i probably overstated my point by saying drywall has no shear strength, but we seem to agree that drywall < plaster & lath[0] < plywood in that respect. i'd suggest you've also overstated the strength of drywall as "significant". it's not nothing, but against an earthquake, it's approximately nothing. plywood backing, of course, is the minimum required by code for earthquake resistance.
plaster & lath is harder to work with, but has many superior qualities (re: moisture, sound, heat, malleability) that it makes it worth it in many residential cases (not so much commercial, where reconfiguration is more frequent).
[0]: a lathe is a machine tool